A head-up display system is an optical display system that employs a combiner element through which an observer views an outside world scene and which reflects visual source information for display to the observer. The source information is typically carried by light emanating from the display surface of a cathode-ray tube. Head-up display systems are typically installed in fighter aircraft so that the pilot can simultaneously monitor critical flight information and observe events occurring outside the aircraft. Conventional combiner elements are of spherical shape and are designed to cooperate with a relay lens to collimate the source information-carrying light so that the display information appears to be coming from optical infinity.
The use of head-up display systems in motor vehicles would likely increase the safe use of and solve space availability problems in such vehicles. In particular, the dashboard area of an automobile has become susceptible to clutter as the trend toward the inclusion of more instrumentation and convenience features continues. The problem of dashboard clutter has heretofore been remedied by the placement of essential control knobs and switches on the steering column. The use of a head-up display system would permit projection of certain vehicle status and performance information, such as engine RPM, vehicle speed, and turn signal indications, forward of the vehicle for display to the driver after reflection from a reflection enhancement material applied to or embedded in the windshield. Projecting such information out in front of the vehicle minimizes the time the driver needs to view the information, thereby enhancing safety in vehicle operation.
A problem inherent in displaying the source information to the observer in this manner is that reflecting light off of a nonplanar surface optically aberrates the light carrying the display information. The surfaces of windshields designed for contemporary automobiles are of complex shapes in that they are typically aspheric with different curvatures in different sections lying in mutually orthogonal planes. The complex shapes stem from aesthetics and from requirements that automobiles exhibit better aerodynamic performance. These requirements dictate that the windshield be inclined at a relatively large slope, i.e., at a "low angle" and be curved near the hood and roof lines of the vehicle so that the windshield forms a complex curvature between its top and bottom margins. The complex shape of the windshield necessitates, therefore, the use of a correction lens having optical light directing properties that compensate for those of the windshield that would otherwise aberrate light carrying the display information.
A problem arising from the need for a correction lens in an automobile head-up display system is that different automobile models or body styles generally require windshields of different profiles. Correction lenses characterized by different optical light directing properties would, therefore, be required to compensate for the optical aberrations introduced by windshield of different profiles.
The windshield must have adequate light transmission properties for the observer to view the outside world scene. Reflection enhancement devices for windshields can be of the wavelength nonselective type, such as a thin metallic or dielectric coating, or of the wavelength selective type, such as a multilayered dielectric coating or a hologram applied to a surface of the windshield or embedded within the laminae of the windshield. A process for embedding a hologram between two laminae of a windshield is described in U.S. Pat. No. 4,842,389 for "Vehicle Display System Using a Holographic Windshield Prepared to Withstand Lamination Process."
The use of a windshield reflection enhancement device imposes certain system design constraints, one of which being the reduction of the photopic transmission of light (i.e., the transmission of light carrying the outside world scene) to the observer. The photopic transmission can be increased by using a wavelength selective device, which reflects light over a narrow bandwidth and thereby maximizes the windshield photopic transmission. The use of a wavelength nonselective device entails a direct tradeoff between display brightness and photopic transmission; whereas, the use of a wavelength selective device offers maximum display brightness and photopic transmission. Another constraint is that the windshield reflection enhancement device be of sufficient quality so that it remains in place and wrinkle free.